Nature-inspired compound

The large-scale production of hydrogen from water and sunlight could be possible with a new catalytic system that takes inspiration from nature.

The five-year research project – sponsored by the Engineering and Physical Sciences Research Council (EPSRC) – will create a compound that mimics the active site of the enzyme organisms used to convert hydrogen ions to hydrogen gas during anaerobic (without oxygen) respiration.

If successful, the project could provide a springboard towards large-scale water photolysis (converting light energy to chemical energy) for a sustainable hydrogen economy.

OxfordUniversity chemist Dr Erwin Reisner, the leader of the effort, has already demonstrated the ability to produce these special enzymes, called hydrogenases, with nickel and iron in their active sites.

Through previous experiments, he showed that shining light on the enzymes, which were steeped in water and a solution that provided electrons and protons, will produce hydrogen.

The process is inspired by bacteria such as E.coli, which naturally possess hydrogenase enzymes. These organisms produce hydrogen as a by-product of normal microbial metabolism.

Reisner now wants to determine if hydrogen production will work with only the active sites of the enzymes.

‘The active site should, in principle, do the same catalysis as the enzyme, but it will be much cheaper,’ he said. ‘The enzymes are expensive and they are a big size so they have a very large footprint. Small molecules would have a much smaller footprint and possibly better longevity.’

The active site will still be based on iron and nickel and surrounded by cysteine (an amino acid), carbon monoxide and cyanide ligands.

‘It shouldn’t be too difficult to get a molecule that looks like the active site, but it’s another story to get it to work and do what you want,’ added Reisner.

The operation of the catalytic system will be observed through spectroscopy techniques provided by project partners Carnegie Mellon University in the US and Manchester University. The results will tell Reisner how to improve its design.

He hopes to have a fully functional catalytic system in five years and he predicts that, within a decade, his efforts could help bring about the large-scale production of hydrogen.

‘I don’t think it’s that far away, simply because there is a lot of effort going into this research area,’ said Reisner. ‘You can see breakthroughs all over the place at the moment.’

For the last several years, researchers around the world have been interested in the use of hydrogenases as a replacement for platinum catalysts. Reisner said this is because the precious metal is limited and costly. He added that the unwanted side reactions from platinum catalysts waste energy and platinum can be poisoned by only trace amounts of common chemicals such as carbon monoxide.

He added that his work with hydrogenases stands out because most people use only iron and no nickel for the enzyme’s active site.

‘Usually, nickel-iron hydrogenases are more robust to air so we’re more likely to have a system that operates under ambient conditions,’ he said. ‘The other type of hydrogenases, which are called iron-iron hydrogenases, are extremely oxygen sensitive, so whenever they are exposed to air they will actually die.’

Reisner added that this would make them too difficult and expensive to use on an industrial scale.

The five-year research programme will focus not only on producing hydrogen but also on wastewater treatment and how living organisms convert water into hydrogen on a molecular level.

Reisner also hopes to reveal how the reverse reaction works – the generation of energy from hydrogen. He believes those results could be important for fuel-cell applications.

Siobhan Wagner

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